Three Electron Reversible Redox Reaction in Sodium Vanadium Chromium Phosphate as a High‐Energy‐Density Cathode for Sodium‐Ion Batteries

Abstract A sodium‐ion battery operating at room temperature is of great interest for large‐scale stationary energy storage because of its intrinsic cost advantage. However, the development of a high capacity cathode with high energy density remains a great challenge. In this work, sodium super ionic conductor‐structured Na 3 V 2− x Cr x (PO 4 ) 3 is achieved through the sol–gel method; Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 is demonstrated to have a capacity of 150 mAh g −1 with reversible three‐electron redox reactions after insertion of a Na + , consistent with the redox couples of V 2+ / 3+ , V 3+ / 4+ , and V 4+ / 5+ . Moreover, a symmetric sodium‐ion full cell utilizing Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 as both the cathode and anode exhibits an excellent rate capability and cyclability with a capacity of 70 mAh g −1 at 1 A g −1 . Ex situ X‐ray diffraction analysis and in situ impedance measurements are performed to reveal the sodium storage mechanism and the structural evolution during cycling.